1. Field of the Invention
The present invention relates to organosiloxane compositions that cure by a hydrosilylation reaction. More particularly, this invention relates to organosiloxane compositions which can be cured by a hydrosilation reaction to yield elastomers exhibiting low compression set values.
This invention also relates to a method for reducing the compression set values of elastomers prepared from a specified class of curable organosiloxane compositions by incorporating a phthalocyanine compound or a metal derivative of said compound into said compositions.
2. Description of the Prior Art
Organosiloxane compositions which cure by a hydrosilylation reaction, i.e. the addition of silicon-bonded hydrogen atoms across silicon-bonded alkenyl radicals, are well known. Elastomers prepared by curing these compositions in the presence of a platinum-containing catalyst are used in components for office automation equipment, automobiles, and other devices because these compositions are easy to process and they cure more rapidly at lower temperatures than organosiloxane compositions that cure by free radical-initiated reactions.
A shortcoming of elastomers prepared from compositions that cure by a platinum-catalyzed hydrosilylation reaction is their relatively high compression set values when measured according standard test methods such as ASTM test method D 395 or Japanese Industry Standard (JIS) test method K 6301. The compression set values are sufficiently high to render the elastomers unsuitable for applications such as gaskets and seals.
Fraleigh et al. [Rubber World (1980) 181(5) 33] report the need for post-curing at 200.degree. C. to reduce the compression set of elastomers prepared from commercially available liquid organosiloxane compositions that cure by a platinum-catalyzed hydrosilylation reaction.
The need to post-cure the cured elastomers by heating them for extended periods of time increases processing costs and considerably reduces the productivity rate for the final elastomer.
Accordingly, there is a need for curable organosiloxane compositions having the rapid cure rate characteristic of hydrosilylation reactions and which yield elastomers exhibiting compression set values below about 20 percent without a lengthy post-cure.
It is known to improve the physical properties, particularly compression set, of elastomers prepared from organosiloxane compositions cured by a platinum-catalyzed hydrosilylation reaction by including in the compositions various additives, including polyorganosiloxanes containing alkenyl hydrocarbon radicals such as vinyl bonded to non-terminal silicon atoms.
The addition of small amounts (less than 0.1 weight percent) of a mixture of an alkali metal salt of hydrogen-containing phosphoric and a hydrogen-containing phosphorus acid to improve the flame retardancy and compression set of silicone rubber prepared from compositions cured using various types of curing agents and/or catalysts is taught in European Patent Application No. 332,785, published on Sep. 20, 1989.
The preparation of cured silicone rubber exhibiting low values of compression set, high tear strength and self-extinguishing properties from curable compositions comprising a polyorganosiloxane exhibiting a degree of polymerization greater that 1000 and containing from 0.01 to 2 mole percent of silicon-bonded vinyl radicals, organohydrogensiloxanes containing at least 2 silicon-bonded hydrogen atoms per molecule, a zero-valent platinum-phosphorus complex as the curing catalyst, a reinforcing silica and sulfur-free carbon black is taught in Japanese patent publication No. 54/53164, which issued on Apr. 26, 1979.
The addition of reaction products of ferric chloride and triphenylamine to peroxide curable organosiloxane compositions to lower the compression set of the cured elastomer is taught in U.S. Pat. No. 3,645,968, which issued to Phillips et al. on Feb. 29, 1972.
U.S. Pat. No. 4,301,056, which issued to Patzke et al. on Nov. 17, 1981 teaches adding calcium hydroxide to peroxide-curable organosiloxane compositions to lower the compression set values and increase the resistance to property degradation in the presence of heated oil of the resultant cured silicone rubber.
Phthalocyanine compounds and metal derivatives of these compounds have been used as pigments in silicone resins and in organosiloxane compositions that are subsequently cured to form elastomers either by the use of organic peroxides and heating or by the reaction of hydroxyl-terminated polydiorganosiloxanes with alkyl silicates or organohydrogensiloxanes. References disclosing this use of phthalocyanine compounds include U.S. Pat. No. 3,364,161 British patent no. 1,470,465; and Japanese patent publication nos. 1/279,981 and 62/223,788.
U.S. Pat. No. 2,723,964 teaches using polynuclear benzenoid compounds such as copper phthalocyanine and ingoid dyes to improve the thermal stability of silicone elastomers cured using organic peroxides.
The use of indanthrene blue, terphenyl, phthalocyanines or metal phthalocyanines to impart a resistance to degradation in the presence of heat and ionizing radiation to specified types of silicone elastomers is taught in British patent no. 893,399. The elastomers are cured using an organic peroxide, sulfur or alkyl silicates in the presence of metal salts of carboxylic acids or amines as catalysts.